This invention relates to the art of interpolators and more particularly to a digital interpolator for providing selective linear interpolation between successive digital data words in a stream of such data words.
The digital interpolator is disclosed in conjunction with a waveform generator with the interpolator being employed to provide interpolation between successive data words that are stored and retrieved from a memory. The invention has other applications and may be used in various situations requiring digital interpolation between successive data words in a stream of such data words.
In the prior art it is known to employ interpolators in waveform generator applications to interpolate between stored waveform samples to provide additional samples at the interpolated values. One such example is that illustrated in Dagostino et al., U.S. Pat. No. 4,263,593. The interpolator therein provides interpolation samples between stored waveform samples to provide additional samples at the interpolated values so as to increase the dot density of a dot density display. The stored samples in digital format are first converted to analog form by digital-to-analog converters and then a transversal filter is employed for obtaining the interpolated values. A similar interpolator for a waveform generator is found in a waveform generator provided by Wavetek of San Diego, Calif. in their Model 175 arbitrary waveform generator. This product is described and illustrated in their instruction manual, instrument release D-2, 1980, manual revision May, 1980. Such prior art achieves interpolation in the analog domain and, hence, this results in reduced bandwidth and lower signal-to-noise ratios, since switching transients have to be suppressed.
An interpolator operating in the digital domain is known in the prior art and takes the form of Candy et al., U.S. Pat. No. 4,313,173. The interpolator therein provides circuitry for computing the difference between successive samples, divides the difference by the number of interpolation intervals to form an increment and then repeatedly adds the increment to the last output of the device in order to generate the next output. This interpolation is continuous and has no ability to provide selective interpolation or no interpolation through an assigned data point or sample.
It is sometimes desirable, such as in generation of waveforms, that the interpolation be selective at designated data points or samples. For example, in generating a waveform where within the waveform it is desired to have an immediate step increase between data points, interpolation therebetween would provide a gradual or ramp function instead of an immediate stepwise function. Digital interpolators, such as that disclosed in the Candy et al. patent, supra, are not capable of performing this function. The interpolators which operate in the analog format discussed hereinbefore have reduced bandwidth and low signal-to-noise ratios because switching transients must be suppressed.